FR2811129A1 - NOISE MITIGATION PANEL, MANUFACTURING METHOD, UNITS INCLUDING THE PANEL - Google Patents

Abstract

An aircraft noise reduction panel (33) includes a cellular core (35) and a facade sheet (36). An array of holes (31) is laser drilled to provide: (i) a variation in the size of the holes in the facade sheet (36); (ii) a non-circular cross section of the holes; (iii) a polygonal cross section of the holes; (iv) holes not contiguous to the walls of the cell nucleus; or (v) inclined holes (31) passing through the facade sheet (36) in a direction inclined relative to normal to the facade sheet (36).

Description

The present invention relates to noise reduction panels and a

method of making a

noise reduction panel.

  Noise reduction panels are widely used to reduce noise produced by aircraft engines and are located in optimized locations in the flow ducts of aircraft engine nacelle structures. Such flow conduits mainly include the inlet conduit, the

  blower and the nozzle assembly.

  A typical noise reduction panel includes

  takes a solid sound-reflecting backing sheet or plate, a perforated metal front sheet or sheet and a honeycomb body or cellular body, which is fixed between the backing sheet and the front sheet and which divides the air into a multiplicity of separate cells. When the noise reduction panel is mounted in a flow duct of an aircraft engine nacelle structure, so that the facade sheet is exposed to acoustic waves produced in the duct, the waves acoustics are subject to three mechanisms which lead to a reduction of the acoustic energy under the effect of its conversion into thermal energy, namely (i) friction in the facade sheet, (ii) loss of pressure when the acoustic pressure waves from the duct penetrate into the cells of the honeycomb structure or of the cell nucleus, and (iii) "reactive" suppression of the direct incident acoustic wave by the wave which is reflected by the reinforcing sheet full, the cell depth of the honeycomb structure being "tuned" to the

frequency required.

  Noise reduction panels are visible.

  dence important from the acoustic point of view but, because of the hostile environment in which they operate, it is also obviously necessary that they have a structural rigidity. Since they are part of the nacelle structure of an aircraft engine, it is important that the complete component is provided with adequate strength to withstand the flight conditions to which an exposed part of a structure is sensitive. with basket. In addition, noise attenuation panels are frequently configured so as to increase the solidity of the basket structure, in which they are to be installed. Perforated facade sheets of panels

  noise reduction measures proposed for this purpose were

  usually drilled by punching or by mechanical drilling. Common structures for noise reduction panels have facade sheets

  perforated with holes typically having a diameter

  meter between 0.508 mm and 1.524 mm, arranged in an equidistant triangular network so as to form

  open areas within the limits of 3 and 20%.

  Previous manufacturing procedures

  included punching and drilling, but does not allow

  were not realistically to form very narrowly spaced holes having very small diameters. In metal sheets, punching, for example, requires a minimum hole diameter of 0.508 mm. Mechanical drilling can produce holes with diameters as small as 0.254 mm, but is almost impossible with holes with the smallest possible diameter.

0.508 mm.

  In document GB 2314526, a method has been proposed for manufacturing a noise attenuation panel, in which a plain facade sheet is pierced by means of an electron beam to produce a multiplicity of perforated holes having diameters not more than 0.508 mm. It is indicated that drilling using an electron beam can also advantageously produce the multiplicity of drilled holes having diameters in the

  range between 0.0508 mm and 0.508 mm.

  It has also been proposed in document US 4850093 to form a perforated titanium facade sheet by laser forming holes or perforations through the sheet. The holes are distributed uniformly on the sheet and occupy 3 to 6% of the total surface of the sheet. It is indicated that the porosity of the sheet can be chosen so as to provide specific resistance to flow either by modifying the size of the holes or the spacing of the holes, or by simultaneously modifying these two parameters. A facade sheet considered suitable had holes between 0.0508 and 0.0762 mm in diameter, hole spacing between 0.203 and 0.406 mm, 11,000 to 16,000 holes per area of 6.45 mm2 and 3 at 6% of

open surface.

  In document GB 2038410A, it has been proposed to manufacture a noise attenuation panel for a flow duct of a gas turbine turbojet engine, which aims to attenuate as many frequencies as possible by means of the use, below the perforated facade sheet, of a Helmholtz type resonator for frequencies situated at the lower limit of the frequency range and of tubular type resonators for higher frequencies. Care is taken to modify the characteristics of the Helmholtz resonators to achieve absorption in a wide band. The facade sheet has a regular array of holes having uniform sizes, although it is proposed to increase the density of the holes by reducing the spacing between the holes at a location in the

  facade sheet to achieve acoustic coupling.

  In document US-A-4288679, a microperforation process is proposed which uses a powerful laser beam and according to which the surface finish and the precision on the dimensions of the hole formed are improved by heating the workpiece. It is indicated that the laser beam can be rotated about its axis to form holes having a surface finish having an accuracy on the dimensions greater than that which can be obtained with a conventional technology of laser microperforation. An object of the present invention is to provide a noise attenuation panel and a method for manufacturing this panel, in which the holes located in the facade sheet can achieve, due to their geometry and their distribution, attenuation of the noise. noise in a wide range of frequencies, to which the panel is subjected, when used as a noise attenuation panel for gas flow conduits in

turbojets.

  In accordance with its different aspects, the pre-

  The present invention relates to a noise reduction panel and to the manufacture of a noise reduction panel which comprises a cellular component which comprises: a front side and a rear side and a wall structure defining cells, which defines a multiplicity of cells between the front face and the rear face, and a facade component which comprises a front face and a rear face, extends across the ends of the cells of the cellular component on the front face of this component, the face rear of the facade component being adjacent to the front face of the cellular component, and is provided with a multiplicity of holes, which pass through the facade component from the front face to the rear face to establish a communication of gaseous fluid between the cells of the cellular component and the front face of the facade component to attenuate the noise produced by the flow of gaseous fluid on the surface of the front face

of the facade component.

  According to a first aspect of the present invention,

  there is provided a method of manufacturing a

  noise reduction of the type indicated above, characterized in that it consists in producing, during a step of manufacturing holes, the multiplicity of holes passing through the facade component in the form of a network of holes having holes having a size which varies in the facade component so as to give an optimal attenuation performance to the panel in a predetermined range of gas flow conditions on the face

front of the facade component.

  According to a second aspect of the present invention,

  there is provided a method of manufacturing a

  noise reduction of the type indicated above, characterized in that it provides for producing, during a hole-forming step, a multiplicity of holes passing through the facade component, in the form of a network of holes having a cross section non-circular cross-section, which is chosen to provide an optimal combination of structural strength and attenuation performance provided by the panel.

  According to a third aspect of the present invention

  tion, there is provided a method of manufacturing a noise reduction panel of the type indicated above, characterized in that the facade component is in the form of a fiber-reinforced composite article, which comprises a component forming a matrix and a fiber reinforcing component incorporated inside the matrix forming component, the fiber reinforcing component comprises networks of fibers, in which the

  fibers of each network extend in a pre-

  determined in the matrix, the predetermined direction of the fibers of each network differs from that of the fibers of each of the other networks, and the holes contained in the facade component are formed so as to have a polygonal cross section provided with sides arranged parallel to the directions network fibers. According to a fourth aspect of the invention, there is provided a method for manufacturing a noise attenuation panel of the type indicated above, characterized in that the structure with walls defining cells comprises walls which define the multiplicity of cells and which end in end portions located in the front face of the cellular component, and the drilling of the holes is carried out in such a way that no hole is drilled at the locations of the facade component which, in the assembled panel, are adjacent to the end portions of the walls

  of the wall structure defining cells.

  According to a fifth aspect of the present invention

  tion, there is provided a method of manufacturing a noise attenuation panel of the type indicated above, characterized in that it plans to form, during a step of forming holes, the multiplicity of holes through the component of facade in the form of a network of holes which pass through the facade component from its front face to its rear face, with a predetermined direction of the holes

  inclined from normal to the front face.

  In one embodiment of the invention, the multiplicity of holes is in the form of a network of holes which pass through the facade component from its rear face to its front face, with a predetermined direction of the holes inclined by compared to normal on the front panel. The inclination of the holes is chosen so as to give the structure defining the cells, flow paths which optimize the performance.

  attenuation provided by the panel.

  When the facade component is a multiple ply structure comprising a plurality of overlapping elements in the form of plies, the holes may be tilted so as to compensate for the structural weakness of the multiple ply structure in the region of the holes. When the panel is arranged so as to be subjected to a gaseous fluid flow on the surface of the

  front side of the facade component in a pre-

  determined fluid flow, the direction predeter-

  mined holes have a component along the front face of the facade component, which is identical to the predetermined direction of fluid flow. Furthermore, the holes can be inclined so that their tendency to be obstructed by debris entrained by the current of gaseous fluid flowing on the front face of the facade component,

be reduced.

  According to a sixth aspect of the present invention, there is provided a noise reduction panel as indicated above, which is manufactured by means of the

  process according to the first, second, third and / or qua-

third aspects.

  According to a seventh aspect of the invention, there is provided a noise attenuation panel of the type indicated above, which is characterized in that the multiplicity of holes passing through the facade component forms a network of holes having a size which varies in the facade component so as to provide optimum attenuation performance by the panel, in a predetermined range of gas current conditions on the face

front of the facade component.

  According to an eighth aspect of the present invention

  tion, there is provided a noise reduction panel as indicated above, which is characterized in that the multiplicity of holes passing through the facade component forms a network of holes having a non-circular cross section, which is chosen so to provide an optimal combination of structural strength and

  attenuation performance provided by the panel.

  In one embodiment of the invention, the holes of the network have a cross section not

  circular which varies in the facade component.

  Furthermore in one embodiment of

  the invention, the holes in the network are spaced by a space

  cement which varies in the facade component.

  In embodiments of the invention which will be described below, the holes of the facade component

  are formed by laser drilling.

  In one embodiment of the invention, the method includes the joining of two components during a

  assembly step during the formation of the

  noise reduction, and performing the hole-forming step before the assembly step. Laser drilling

  is then executed using a high intensity laser

  which can take the form of a CO2 laser or a

YAG laser.

  In one embodiment of the invention, the method includes bringing the two components together during an assembly step when forming the noise reduction panel and performing the hole production step after the assembly stage. Laser drilling

  is then performed using a low intensity laser

  sity, which can take the form of a laser excimer to

ultraviolet.

  In another embodiment of the invention

  tion, the panel further comprises a reinforcing component extending over the rear face of the cellular component, and the assembly step provides for joining the three components

  when forming a noise reduction panel.

  In one embodiment of the invention in accordance with the fourth aspect of the invention, the method includes joining the two components during an assembly step when forming the noise reduction panel, performing the hole formation step after the assembly step and the execution of laser drilling under the control of an ultrasonic probe identifying

  the locations of the end portions of the walls.

  In one embodiment according to the invention, the network holes have drilling patterns which reduce blockage due to debris deposited by the

  gas flow through the holes.

  In a ninth aspect, a noise abatement panel of the type indicated above is provided, in which the facade component is in the form of a fiber-reinforced composite article, which comprises a matrix component and a component. fiber reinforcement incorporated within the matrix component, the fiber reinforcement component having fiber arrays, in which the fibers

  of each network extend in a pre-

  determined in the matrix, and the predetermined direction of the fibers of each network differs from that of the fibers of each of the other networks, and the holes contained in the facade component are formed so as to have a polygonal cross section provided with sides arranged parallel to the predetermined directions of the fibers of the networks.

  When the reinforcing fiber networks include

  take first, second, third and fourth networks of fibers whose fibers extend in directions 0, 90 and +45 and -45, the holes in the facade component have an octagonal cross section whose sides are parallel to the directions

fibers.

  When the reinforcing fiber networks include

  take first and second fiber networks, the fibers of which have directions 0 and 90, or +45 and -45, the holes formed in the facade component are produced so as to have a polygonal cross section with four sides, the sides are parallel to

fiber directions.

  According to a tenth aspect of the invention, there is provided a noise attenuation panel as indicated above, in which no hole is provided at the

  facade component locations, which when the pan-

  neau is assembled, are contiguous to the end parts

  walls of the wall structure defining cells

  Lules. According to the eleventh aspect of the present invention, there is provided a noise reduction panel as indicated above, in which the multiplicity of holes passing through the facade component is in the form of a network of holes which pass through the façade component from its rear face to its front face, with a predetermined direction of the holes, inclined relative to

normal to the front panel.

  The multiplicity of holes passing through the facade component is preferably in the form of a network of holes which pass through the facade component from its rear face to its front face, with a predetermined direction of the holes, inclined relative to normal to the front panel. The inclination of the holes is chosen so as to provide flow paths to the structure defining cells which optimize the

  panel attenuation performance.

  When the facade component is a multiple ply structure comprising a plurality of overlapping ply elements, the holes are inclined by

  so as to compensate for the structural weakness of the structure

  ture with multiple plies in the region of the holes.

  When the panel is arranged to be

  subject to a stream of gaseous fluid flowing over the surface

  face of the front face of the facade component with a

  predetermined direction of fluid flow, the direction

  The predetermined hole has a component along the front face of the facade component, which is identical to the predetermined direction of flow of the fluid. In addition, the holes can be tilted so as to reduce the tendency of the latter to be obstructed by debris entrained by the stream of gaseous fluid flowing over the

front of the facade component.

  When the facade component is in the form of a fiber-reinforced composite product comprising a matrix component and a fiber-reinforced reinforcement component incorporated in the matrix component, laser drilling is performed in the composite product after component hardening

forming matrix.

  In one embodiment of the invention, the facade component and the cellular component must follow a predetermined outline for the panel when assembled, and the pre-hardening of the facade component is carried out with the predetermined outline for the panel. In addition, panels produced in accordance with the invention may include facade components provided with holes having two or more of two characteristics required in the different aspects.

  previously mentioned of the invention.

  The invention will be better understood on reading

  the description which follows, given only as

  non-limiting example, with reference to the accompanying drawings in which: Figure 1 shows a schematic section of an aircraft propulsion unit using noise reduction panels;

  - Figure 2 is a schematic perspective view

  tick, from the top, of a noise reduction panel hitherto proposed for use in the propulsion unit shown in FIG. 1; - Figure 3 shows a schematic cross section of part of the panel shown in Figure 2, fixed on a support channel element;

  - Figure 4 shows a schematic representation

  tick of a part of the front sheet of the panel shown in Figures 2 and 3, modified in accordance with the first and seventh aspects of the invention;

  - Figure 5 shows a schematic representation

  tick of a part of the front sheet of the panel shown in Figures 2 and 3, modified in accordance with the second and eighth aspects of the invention;

  - Figure 6 shows a schematic representation

  tick of part of the front sheet of the panel shown in Figures 2 and 3, modified in accordance with the third and ninth aspects of the invention;

  - Figure 7 shows a schematic representation

  tick of part of the front sheet of the panel shown in Figures 2 and 3, modified in accordance with the fourth and tenth aspects of the invention; and

  - Figure 8 shows a schematic representation

  tick of part of the facade sheet of the panel shown in Figures 2 and 3, modified in accordance with

  fifth and eleventh aspects of the invention.

  Referring first to Figure 1, the propulsion unit shown comprises an engine body 11 carrying fan blades 2 and surrounded by a nacelle structure 12 of the engine, which forms an annular fan duct 13 serving to drive a gas flow of blower at high speed towards

  an annular outlet nozzle 10.

  As can be seen, the basket structure 12 comprises

  door, at its front end, an inlet cover 14 provided with noise attenuation panels 15 as previously proposed and arranged as will be

  described below with reference to Figures 2 and 3.

  The pod structure 12 further includes, at its rear end, a thrust reverser unit 16 shown in a retracted position in the upper half of Figure 1 and in the deployed position in the lower half of Figure 1. L the reverse thrust unit 16 is an integral part of the flow stream

  blower 13 and exhaust nozzle 10.

  To reduce the noise emanating from the

  fan in the region of the reverse thrust unit

  sée, the inner wall 26 of the cover 24 and the inner wall

  32 of the fan duct are provided with pan-

  noise reduction levels 17 and 18, which can also

  be in the form of a noise reduction panel

  which will be described with reference to Figures 2 and 3.

  Referring now to Figures 2 and 3, a noise reduction panel 33, as previously proposed, includes a backing sheet 34, a core

  cell 35 and a facade sheet 36.

  The cell nucleus 35 comprises a multiplicity of juxtaposed cells 37 with open ends having a hexagonal cross section serving to form a

  honeycomb configuration. Otherwise, we can naturally-

  use cell nuclei with cells

  juxtaposed lules having other cross sections.

  The reinforcing sheet 34 is not perforated and is formed of a permeable sheet-like material and is fixed by an adhesive to the underside of the cell core 35. The facade sheet 36 is fixed to the top face of the core cell 35 also using a

  adhesive. The cells 37 are provided with drainage slots.

  swim 40 allowing evacuation of the condensate out of the

component 33.

  The support structure for the panels 33 usually includes support elements in the form of U-profiles, of which only one is shown in FIG. 3. The panel 33 is fixed to the element 41 by fixing the sheet of facade 36 to an outer face of a flange 42 of the element formed by a U-shaped profile 41 by means of the use of an adhesive 43 and by fixing the reinforcement sheet 34 to the outer face of a flange

  44 of the U-shaped element 41 by means of the use

adhesive 45.

  The curved panels 33 having even

  tuuel a double curvature, are arranged in the form of structural parts in one or more locations, 17 and 18 of the motor shown in Figure 1, each component being one of several curved components which extend circumferentially around the structure to

nacelle.

  As shown in FIGS. 2 and 3, the front sheet 36 is provided with a multiplicity of holes 31 which establish a communication for a gaseous fluid between the cells 37 of the cell nucleus 35 and the front face of the sheet 36. As described above, the holes 31 in the facade sheet 36 of the panel 33 had hitherto had a circular cross section, of uniform size over the surface of the facade sheet 36 and were distributed uniformly over the surface of the facade sheet 36. The holes 31 were formed by conventional mechanical drilling, drilling using a laser beam or drilling using an electron beam before assembly of the panel 33, that is to say before the meeting step reinforcement sheet 34, cell nucleus 35

and facade sheet 36.

  To obtain noise attenuation in a wide range of frequencies, the geometry and the distribution of the holes 31 are modified in accordance with one or more

  of the various aspects of the present invention.

  In particular, in accordance with the first and sep-

  tth aspects of the invention and as shown in FIG. 4, holes 31 in the facade sheet 36 have the form of a network of holes whose size varies in the facade sheet 36. The variation in size holes are chosen so as to give the

  panel optimum attenuation performance.

  In addition or otherwise, the holes 31 passing through the facade sheet 36 are arranged in accordance with the second and eighth aspects of the present invention and are as shown in FIG. 5 in the form of a network of holes having a non-circular cross section,

  or include such an array of holes.

  Furthermore, in accordance with the third and ninth aspects of the invention, the facade sheet 36 is in the form of a fiber-reinforced composite product comprising a matrix component and a fiber-reinforced reinforcement component which is housed in the matrix component. The reinforcing component formed of fibers comprises networks of fibers, in which the fibers of each network extend in a predetermined direction in the matrix, and in which the predetermined direction of the fibers of each network differs from that of the fibers of each of other networks. The holes 31 located in the front sheet 36 are then formed so as to have a polygonal cross section with sides arranged parallel to the directions

  network fibers.

  As shown in Figure 6, the

  facade sheet 36 has the form of a composite product

  fiber reinforced site comprising a component

  mant matrix 136 and a reinforcing component formed of fibers 137 containing networks of fibers 138, 139, 140 and 141 with directions of fibers at 0, 90 and +45 and -45, while the holes located in the facade sheet are arranged so as to have an octagonal cross section, the sides of which are arranged parallel to the

fiber directions.

  In accordance with the fourth and tenth aspects of the invention and as shown in FIG. 7, the holes 31 are formed in the front sheet 36 so that no hole appears at the locations of the front sheet 36 which , in the assembled panel 33, are contiguous with the end portions of the walls 135 of the

  cell nucleus structure 35.

  According to the fifth and tenth aspects of the invention and as shown in FIG. 8, the multiplicity of holes 31 passing through the front sheet 36 has the form of a network of holes which cross the front sheet 36 from the face rear adjacent to the front face of the cell core structure 35 to the front face 361 of the facade sheet 36, the holes extending in an inclined direction H, as is

  shown, relative to normal on the front face 361.

  The panel 33 as described above with reference to Figures 2 and 3 and to Figure 8 is located in the propulsion unit shown in Figure 1 at any of the three locations indicated by the panels 15, 17 and 18 and o the panel is subjected to a gas current circulating on the surface of the front face

  361 of the facade sheet 36 in a direction of

  rant of fluid identified by arrow F in FIG. 8.

  In the embodiment of the invention shown in Figure 8, it is expected that the direction H of the holes has a component extending along the front face 361 of the facade sheet 36, which extends in the even

  direction as direction F of the fluid stream.

  Appropriate construction and manufacturing cycle of a noise reduction panel according to

  the invention according to its various aspects, will be indi-

as below.

  Construction of the panel (1) Reinforcement sheet Reinforcement: carbon (woven fabric, unidirectional, not crimped),

  Resin system: thermosetting or thermoplastic-

tick

  Layout: isotropic, quasi-isotropic, orthogonal.

  Typical construction: 3K woven carbon fibers

  according to a configuration with 8 skeins and epo matrix

  xy (3-4 layers with a thickness between 0.114 cm-

  0.152 cm) (2) Honeycomb core

  Types of cores: (i) nomex (aramid), (ii) glass-

  forced by fibers and immersed in a phenolic resin

  league, (iii) metallic, for example aluminum alloy (An insulating layer can be used between the metallic core and the component formed from the carbon-based composite material) (3) Facade sheet Perforation performed by laser: hot laser (CO2 YAG); cold laser (ultraviolet excimer) Reinforcement: carbon or glass or aramid (woven or unidirectional or non-crimped fabric)

  Resin system: thermosetting or thermoplastic

  as Arrangement: isotropic, quasi-isotropic, orthogonal Typical construction: 3K carbon fibers woven in an 8-skein configuration and epoxy matrix

  (2 to 3 tablecloths, thickness 0.081 cm-0.114 cm).

  Laser drilling is performed on a laminate

  previously hardened, which has the outline of the compound

being finished.

  Manufacturing cycle Scenario 1 (drilling with a "hot" and "cold" laser) (1) Pre-hardening of (pre-impregnated layers of the reinforcement sheet (consolidated / hardened (2) Pre-hardening of (pressurized the front sheet (at the autoclave in accordance with the profile (3) Piercing of the facade sheet by laser (4) Crosslinking of the adhesive on the honeycomb core or on the perforated facade sheet (5) Assembly of the reinforcement sheet, of the honeycomb core and the facade sheet and meeting of these

elements between them.

  Scenario 2 (only "cold" laser) (1) Pre-hardening of (pre-impregnated layers of the reinforcement sheet (consolidated / hardened (2) Pre-hardening of (pressurized the facade sheet (in the autoclave according to the profile (3) Assembly of the reinforcement sheet, the honeycomb core and the facade sheet and assembly of these

elements between them.

  (4) Laser perforation of the facade sheet in the assembled assembly

  (The honeycomb core is attached to the cover sheet.

  strong by means of an adhesive film. The core where the facade sheet is cross-linked with the adhesive to facilitate bonding at the honeycomb / sheet interface

facade).

  The obvious benefits and advantages provided by pan-

  neaux arranged with facade sheets having a geometry and a distribution of holes in accordance with

  the invention in its various aspects are indicated below

  after. (1) Adaptability regarding the geometry of the holes (use of a "hot" laser and a "cold" laser)

  Simple perforation training in coatings

  holes, typically having a diameter of 0.102 cm with a high NLF factor (nonlinearity factor), which means that their attenuation performance while being good from a design point of view, degrades slightly in other operating conditions. Linear coatings with a very large number of very small holes provide a surface impedance which is invariable when conditions in the conduit vary. Linear coatings are usually found to work best under a wide range of working conditions.

engine operation.

  Coating design studies have shown that the optimum non-linearity factor lies somewhere between the two extremes, although it is closer to the linear coating than simple perforated coating. The improved perforated coating with very small holes produced by any means

  rather than using a trellis (construct-

  standard (open carbon armor and stainless steel mesh) provides the ability to target

  the optimum NLF factor for any application

particular tion.

  Linear coatings formed by a wire mesh on a support sheet are likely to be the site of corrosion, detachment of the wire mesh, poor aesthetic appearance; and also require uniform properties, etc. The perforated coating offered has a number of advantages in that it simplifies manufacturing and provides better quality and a more robust coating. Coating design studies have also indicated an acoustic advantage due to variable impedance properties (i.e. the open area across the facade sheet). An analysis also shows an acoustic advantage which can be obtained from a distributed three-dimensional coating. A complex variation of the geometry of the holes and their distribution in the facade sheet is possible with the perforated coating, which in itself leads to

  consequent to a very flexible design of the coating.

  Variations in the geometry of the holes and their

  distribution in the facade sheet lead also

  also with the following advantages: (i) modification of the shape of the holes - the shape of the holes can be specially adapted for any given open surface in order to maximize the structural rigidity (for example an octagonal shape for a configuration

  quasi-isotropic (see Figure 6), a rec-

  tangular for an orthogonal arrangement) and acoustic performance. (ii) Variable surface and spacing of the holes - this achieves the adjustment of the open surface in accordance with the acoustic requirement. Hole size and spacing may vary by

  panel to another, and also in a panel.

  This provides an opportunity to achieve very

  small holes allowing a perforation

  liorée. (iii) The geometry of the holes can be narrowed over the thickness so as to reduce the effects of

  blocking and provides a self-cleaning mechanism.

  (2) Piercing at a fixed depth ("cold" laser only) It is possible with the "cold" ultraviolet excimer laser to pierce the perforations in the facade sheet after the bonding operation (i.e. the previously hardened reinforcement sheet, the honeycomb core and the laminate of the facade sheet are joined together and the finished component is

  then pierced by laser (see scenario 2 above).

  Fixed depth drilling provides the following production advantages: - perforation technique not implemented in a critical path no crosslinking process necessary

  - can treat a three-dimensional curvature (even-

* only when using a universal fastening device) - better use of the material - possibility of total automation, i.e. perforation with quality guarantee systems

and / or online self-diagnosis.

  Fixed depth perforation provides the following advantages / aesthetic advantages: - no resin leakage from the surface of the facade sheet

  - possibility of painting the front sheet of the pro-

  composite composite according to customer requirements, - possibility of applying (organic) coatings

  resistant to abrasion or erosion.

  The fixed depth perforation process (only "cold" laser) can also make it possible to identify / predict (by means of, for example, variations in depth, the ultrasonic probe, visual means) the locations of the cell walls in honeycomb (after the bonding of the reinforcement sheet, the facade sheet and the honeycomb core) and to drill the holes in the cell. Holes do not have to coincide with the cell wall, which blocks them acoustically. In this way the panel is more structural since it has fewer holes for the open area required. This is due to the tighter tolerance on the open surface (and therefore the acoustic performance), which can be obtained since it is not necessary to make arrangements to allow a hole blocking caused during the bonding under the effect of adhesive leakage or alignment of the

honeycomb cell.

  Although it may be appropriate to use an ultrasonic probe to detect the locations of the walls of the honeycomb cells (after fixing the backing sheet, the facade sheet and the honeycomb core) note that we can also

  use other non-destructive detection techniques.

  Such non-destructive detection techniques with potential for detecting honeycomb cell walls through the facade sheet include: 1. visual systems

  2. the techniques identified as being based mainly on

  basics on ultrasound and including: (i) laser coupled systems (non-contact) (ii) pneumatic coupling systems (non-contact) (iii) Acoustography - ("real time" immersion systems with large surface area) (iv) Filmless radiography (v) Transducers with multiple networks (applications in service) Among the new suitable techniques one

  counts: microwaves; tomography; thermogra-

  phy; RTUIS @ Dassault; acoustic doppler - RAID; ultrasonic video process - Acoustocam; ANDSCAN - (in use) and

MAUS - (in use).

  The advantages that can be obtained with the use

  sation of the present invention in accordance with its various aspects include the fact that:

  1. The hot laser can be used to form per-

  holes in the facade sheet before assembly or when the honeycomb core is attached to this

  sheet provided that a system that can detect pa-

  kings of honeycomb cells either in function-

  ment. 2. The ability to drill a hole in the center of each cell by detecting the walls of the honeycomb cells is optimal both from an acoustic point of view and from a structural point of view; 3. Regardless of whether or not the honeycomb cell walls can be detected, the

  cutting of holes in the complete linked assembly in use

  Reading the excimer laser has the advantages of avoiding the crosslinking step and of avoiding an unsightly leakage of the adhesive together with all the other advantages mentioned, including for example the drilling of facade sheets, which are

  painted or provided with a protective coating.

  (4) The use of the excimer laser makes it possible to drill / uncover

  to drill a hole over a known predetermined depth so that the honeycomb or the adhesive located

  underneath the facade sheet is not damaged.

  Likewise this technique (unlike other hot laser techniques) does not provide any "heat affected area" (an area adjacent to the periphery of the hole, where the heat from the laser has destroyed the resin matrix) and by therefore provides a facade sheet with characteristics

structural improvements.

  (5) For a specified open perforated surface (determined acoustically for an aircraft engine), the most structural facade sheet is obtained by

  perforation of perforations using the exci-

  mother, with a geometry of holes which is adapted to the orientations of arrangements of the fibers of the facade sheet and which comprises holes formed in the center of

  the cell, away from cell walls.

  Laser parameters To obtain a high quality of holes and fast processing speeds, it is necessary to determine the optimal combination of parameters such as the laser wavelength, the repetition frequency (cutting speed), the length of the pulses, energy

and the drilling technique.

  The following drilling techniques are available: (i) Percussion drilling During percussion drilling, the laser beam, which is used to create the hole, has almost the same size as the drilled hole. This is obtained by controlling the profile of the focused beam or by forming images with

mask projection.

  (ii) Rotational drilling During rotary drilling, the laser beam is significantly smaller than the drilled hole and is swept around the circumference of the hole so as to allow the material in the center of the hole to be removed. The hole can be created using a single pass of the laser or by means of a number of passes. Likewise, the holes can be drilled one by one (single-hole trepanning) or else a network of holes can be made by jointly rotating the laser moving between the holes after each pass of the laser beam (formation of a

network of holes by rotation).

  During manufacturing, the goal is to maximize the rate of perforation. This is achieved by minimizing the amount of material removed by the laser. In hammer drilling, energy from the laser is used to remove all of the material contained in the volume of the hole. During rotary drilling, only a small ring of material is removed and the rate of perforation is therefore higher. In addition, the shape of the hole can be changed in a simpler way and the modification can be easier. Therefore rotary drilling is preferred,

  but we can use both techniques.

  Whenever possible, the heat produced during laser perforation should be able to dissipate, i.e. should be prevented from accumulating to the level which would lead to the appearance of heat affected areas (HAZ), where the resin is burnt and missing and where fibers of the composite material are damaged around the hole. Normally, operation with a laser having an ultraviolet wavelength (200-400 nm) would be the best choice, being

  since this would produce the minimum amount of heat.

  However, the cutting speed is too low for production. Another way to reduce heat is to use laser operation at a visible wavelength or in the infrared, but to choose a high pulse of laser energy and keep a

  mobility of the laser beam at high speed.

  Below, we will indicate preferred types of

  lasers / energies and repetition speeds.

  Preferred systems Typical wavelength A - XeCL gas excimene - 308 nm High energy per pulse - Low repetition rate

B- Solid state - Nd: YAG -

  pumped by flash lamp 355, 532 & 1064 nm

  High energy per pulse - low repetition frequency

tion.

C- Solid state - Nd: YAG -

  355 nm diode pumped Low pulse energy - high repetition frequency.

D- Solid state - Nd: YV04 -

  diode pumped 1064 nm Low pulse energy - repetition frequency

high.

  Hot lasers such as CO2 lasers can

  They are also used because of their high cutting speeds. However, they show areas affected by heat and they cause a reduction

possible product quality.

  It will be noted that apart from facilitating the size and the shape of the holes, the use of a laser to produce the perforations also makes it possible to cut the holes at a certain angle relative to the surface of the facade component of the panel, in practical goals up to a 45 degree angle. This could have the following advantages: Acoustics: Improved attenuation due to the creation of a

  more complex flow path to Helm- tubes

  cell structure holtz (honeycomb). Aerodynamics: Reduction of any drag effect produced

by the air flow.

  Structural: By compensating for the weakness of the structure

  ture at the level of the hole, tablecloth by tablecloth.

  Obstruction: Reduction of the tendency to obstruct the holes by aligning the cutting axis with the air flow. Drilling inclined holes, however, increases the production time of the hole and increases the risk

edge damage.

  It will be noted that the panels formed in accordance with the invention may include a facade sheet comprising two or more of two hole characteristics,

  previously described with reference to Figures 4 to 6.

Claims (53)

  1. Method for manufacturing a mitigation panel   tion of noise, which comprises: - a cellular component (35) which comprises: a front face, a rear face and a wall structure defining cells (37), which defines a multiplicity of cells between the front face and the rear face , and - a front component (36) which comprises a front face and a rear face, extends across the ends of the cells of the cellular component (35) on the front face of this component, the rear face of the front component being adjacent to the front face of the cellular component, and is provided with a multiplicity of holes (31), which   pass through the facade component from the front side to   than on the rear face to establish a gaseous fluid communication between the cells of the cellular component and the front face of the facade component to attenuate the noise produced by the flow of gaseous fluid on the surface of the front face of the facade component , characterized in that the method consists in producing, during a hole manufacturing step (31), the multiplicity of holes passing through the facade component in the form of a network of holes having holes having a size which varies in the facade component so as to give optimum attenuation performance to the panel in a predetermined range of gas flow conditions on   the front face of the facade component.   2. Method for manufacturing a mitigation panel   noise, which comprises: - a cellular component (35) which comprises: a front face and a rear face and a wall structure defining cells (37), which defines a multiplicity of cells between the front face and the rear face , and - a front component (36) which comprises a front face and a rear face, extends across the ends of the cells of the cellular component on the front face of this component, the rear face of the front component being adjacent to the front face of the cellular component, and is provided with a multiplicity of holes (31), which   pass through the facade component from the front side to   only on the rear face to establish a gaseous fluid communication between the cells of the cellular component and the front face of the facade component to attenuate the noise produced by the flow of gaseous fluid on the surface of the front face of the facade component , characterized in that the method consists in producing, during a hole-forming step (31), a multiplicity of holes passing through the facade component, in the form of a network of holes having a non-circular cross section, which is chosen to provide an optimal combination of structural strength and   attenuation performance provided by the panel.   3. Method according to claim 1, characterized in that the holes (31) of the network have a non-circular cross section, which is chosen so as to provide an optimal combination of structural strength   and attenuation performance provided by the panel.   4. Method according to either of the claims   tions 2 and 3, characterized in that the holes (31) of the network have a non-circular cross section,   which varies in the facade component.   5. Method according to any one of the claims   1 to 4, characterized in that the holes (31) of the network are separated by a spacing which varies in the component of facade.   6. Method for manufacturing a mitigation panel   noise, which comprises: - a cellular component (35) which comprises: a front face and a rear face and a wall structure defining cells (37), which defines a multiplicity of cells between the front face and the rear face , and - a front component (36) which comprises a front face and a rear face, extends across the ends of the cells of the cellular component on the front face of this component, the rear face of the front component being adjacent to the front face of the cellular component, and 15. is provided with a multiplicity of holes (31), which   pass through the facade component from the front side to   only on the rear face to establish a gaseous fluid communication between the cells of the cellular component and the front face of the facade component to attenuate the noise produced by the flow of gaseous fluid on the surface of the front face of the facade component , characterized in that the method consists in producing, during the step of producing holes (31), the multiplicity of holes through the facade component in the form of a network of holes which pass through the component from the face rear of the facade component to the front face of the facade component, in a predetermined direction of the holes, which is   inclined from normal to the front face.   7. Method according to any one of the claims   1 to 5, characterized in that the multiplicity of holes (31) in the facade component is in the form of a network of holes which pass through the facade component from its rear face to its front face, with a predetermined direction of holes inclined with respect to normal to the front panel.   8. Method according to either of the claims   tions 6 and 7, characterized in that the inclination of the holes (31) is chosen so as to provide flow paths towards the structure defining cells, which optimizes the attenuation performance provided by the panel.   9. Method according to claim 8, characterized in that the facade component (36) is a structure with   multiple layers comprising a plurality of elements   mant superimposed tablecloths, and that the holes are inclined by   so as to compensate for the structural weakness of the structure   ture with multiple plies in the region of the holes.   10. Method according to any one of the claims.   cations 6 to 9, characterized in that the panel (15, 17, 18) is located so as to be subjected to a stream of gaseous fluid on the surface of the front face of the facade component (36), with a predetermined direction of the fluid current, and that the predetermined direction of the holes (31) has a component along the front face of the facade component, which extends in the same direction as the   predetermined direction of fluid flow.   11. Method according to either of the claims   cations 6 and 7, characterized in that the holes (31) are inclined so that their tendency to be obstructed by debris entrained by the stream of gaseous fluid   on the front of the facade component is reduced.   12. Method according to any one of the claims.   cations 1 to 11, characterized in that the holes (31) located in the facade component are formed by laser drilling.   13. Method according to any one of the claims   cations 1 to 12, characterized in that it provides for: placing the two components (35, 36) against each other during an assembly step during the formation of the noise reduction panel , and implement the hole production step (31) before the assembly step.   14. Method according to claims 12 and 13   taken as a whole, characterized in that the laser drilling is carried out using a laser having a high intensity.   15. The method of claim 14, character-   laughed at in that the high intensity laser is a laser at C02 or a YAG laser.   16. Method according to any one of the claims.   cations 1 to 12, characterized in that it provides: to bring the two components (35, 36) against each other during an assembly step, during the formation of the attenuation panel noise, and performing the hole forming step (31) after the assembly step.   17. Method according to claims 12 and 16   taken as a whole, characterized in that the laser drilling is carried out using a low intensity laser.   18. The method of claim 12, character-   laughed at that the low intensity laser is a laser excimer to ultraviolet.   19. Method according to any one of the claims   cations 1 to 18, characterized in that the panel further comprises a reinforcing component which extends over the rear face of the cellular component and that the assembly step consists in bringing the three components together when   the formation of the noise reduction panel.   20. Method for manufacturing a mitigation panel   tion of noise, which comprises: - a cellular component (35) which comprises a front face and a rear face and a wall structure defining cells (37), which defines a multiplicity of cells between the front face and the rear face and which ends in end portions situated on the front face of the cellular component, and - a front component (36) which comprises a front face and a rear face, extends across the ends of the cells of the cellular component on the front face of this component, the rear face of the facade component being adjacent to the front face of the cellular component, and is provided with a multiplicity of holes (31), which   pass through the facade component from the front side to   only on the rear face to establish a gaseous fluid communication between the cells of the cellular component and the front face of the facade component to attenuate the noise produced by the flow of gaseous fluid on the surface of the front face of the facade component , characterized in that the laser drilling is carried out so that no hole (31) is drilled in locations of the facade component which, in the assembled panel, are contiguous with the end portions of the walls of the structure with cell walls.   21. Method according to any one of the claims   cations 1 to 19, characterized in that the wall structure defining cells (37) comprises walls which define the multiplicity of cells and which terminate in the end portions on the front face of the cell component, and the drilling by laser is executed in such a way that no hole is drilled at locations of the facade component which, in the assembled panel, are contiguous with the end portions of the walls of the cell wall structure.   22. Method according to either of the claims   cations 20 and 21, characterized in that the method provides for: bringing the two components (35, 36) together for a   assembly step, during the formation of the   noise reduction, perform the hole formation step (31) after the assembly step, and perform the laser drilling under the control of an ultrasonic probe identifying the locations of the parts end of the walls.   23. Method according to any one of the claims   cations 1 to 22, characterized in that the holes (31) in the network have drilling patterns which reduce blockage by debris deposited by the current gas passing through them.   24. Process for manufacturing a mitigation panel   noise, which comprises: - a cellular component (35) which comprises: a front face and a rear face and a wall structure defining cells (37), which defines a multiplicity of cells between the front face and the rear face , and - a front component (36) which comprises a front face and a rear face, extends across the ends of the cells of the cellular component on the front face of this component, the rear face of the front component being adjacent to the front face of the cellular component, and is provided with a multiplicity of holes (31), which   pass through the facade component from the front side to   than on the rear face to establish a gaseous fluid communication between the cells of the cellular component and the front face of the facade component to attenuate the noise produced by the flow of gaseous fluid on the surface of the front face of the facade component , characterized in that the facade component (35) is in the form of a fiber reinforced composite article, which comprises a matrix component (136) and a fiber reinforcement component (137) incorporated in the interior of the matrix component, the fiber reinforcing component having fiber arrays (138-141), in which the fibers of each network extend in a predetermined direction in the matrix, and the predetermined direction of the fibers of each network differs from that of the fibers of each of the other networks, and the holes (31) contained in the facade component are formed so as to have a polygo cross section nale provided with sides arranged parallel to the predetermined directions of the fibers of the networks.   25. Process according to any one of the claims.   cations 1 to 23, characterized in that the facade component (36) is in the form of a fiber-reinforced composite article, which comprises a matrix component (136) and a fiber reinforcement component (137) incorporated within the matrix component, the fiber reinforcing component (137) having fiber arrays, in which the fibers of each array extend in a predetermined direction in the matrix, and the predetermined direction of the fibers each network differs from that of the fibers of each of the other networks, and the holes (31) contained in the facade component are formed so as to have a polygonal cross section provided with sides arranged parallel to the predetermined directions of the fibers of the networks.   26. Method according to either of the claims   cations 24 and 25, characterized in that the reinforcing fiber networks (138-141) comprise first, second, third and fourth fiber networks, the fibers of which extend in the directions 0, 90 and +45 and - and that the holes in the facade component have an octagonal cross section whose sides are   parallel to the directions of the fibers.   27. Method according to either of the claims   cations 24 and 25, characterized in that the reinforcing fiber networks (139-141) comprise first and second fiber networks, the fibers of which have directions 0 and 90, or +45 and -45 and that the holes formed in the facade component are produced so as to have a polygonal cross section with four sides, the sides of which are parallel to the directions of the fibers.   28. Method according to any one of the claims   cations 1 to 27, characterized in that the facade component (36) is in the form of a fiber-reinforced composite product and comprising a matrix component (136) and a fiber-reinforced reinforcement component (137) , which is incorporated into the matrix component, and laser drilling is performed on the composite product after the matrix component has been previously cured.   29. Method according to claim 28, character-   rised in that the facade component (36) and the cellular component (35) must follow a predetermined contour for the panel, when they are assembled, and the pre-curing of the facade component is   executed with the predetermined outline for the panel.   30. Noise reduction panel, which includes   door: - a cellular component (35) which comprises: a front face and a rear face and a wall structure defining cells (37), which defines a multiplicity of cells between the front face and the rear face, and - a component facade (36) which comprises a front face and a rear face, extends across the ends of the cells of the cellular component on the front face of this component, the rear face of the facade component being adjacent to the front face of the component cell, and is provided with a multiplicity of holes (31), which   pass through the facade component from the front side to   than on the rear face to establish a gaseous fluid communication between the cells of the cellular component and the front face of the facade component to attenuate the noise produced by the flow of gaseous fluid on the surface of the front face of the facade component , characterized in that the multiplicity of holes (31) passing through the facade component forms a network of holes having a size which varies in the facade component so as to confer optimum attenuation performance on the panel under a predetermined range of conditions flow   gas on the front of the facade component.   - 31. Noise reduction panel, which includes   door: - a cellular component (35) which comprises: a front face and a rear face and a wall structure defining cells (37), which defines a multiplicity of cells between the front face and the rear face, and - a component facade (36) which comprises a front face and a rear face, extends across the ends of the cells of the cellular component on the front face of this component, the rear face of the facade component being adjacent to the front face of the component cell, and is provided with a multiplicity of holes (31), which   pass through the facade component from the front side to   only on the rear face to establish a gaseous fluid communication between the cells of the cellular component and the front face of the facade component to attenuate the noise produced by the flow of gaseous fluid on the surface of the front face of the facade component , characterized in that the multiplicity of holes (31) passing through the facade component form a network of holes having a non-circular cross section, which is chosen so as to provide an optimal combination of structural strength and performance d mitigation provided by the panel.   32. Noise reduction panel according to claim 30, characterized in that the multiplicity of holes (31) passing through the facade component forms a network of holes having a non-circular cross section, which is chosen so as to provide a combination structural strength and   attenuation performance provided by the panel.   33. Panel according to either of the claims   cations 31 and 32, characterized in that the holes (31) of the network have a non-circular cross section which is chosen so as to provide an optimal combination of structural strength and performance attenuation for the panel.   34. Panel according to either of the claims   cations 31 and 32, characterized in that the holes (31) of the network have a non-circular cross section   which varies in the facade component.   35. Panel according to any one of the claims.   cations 32 to 34, characterized in that the holes (31) of the network are separated by a spacing which varies in the facade component.   36. Noise reduction panel which comprises: - a cellular component (35) which comprises: a front face and a rear face and a wall structure defining cells (37), which defines a multiplicity of cells between the front face and the rear face, and - a front component (36) which comprises a front face and a rear face, extends across the ends of the cells of the cellular component on the front face of this component, the rear face of the component facade being adjacent to the front face of the cellular component, and is provided with a multiplicity of holes (31), which   pass through the facade component from the front side to   only on the rear face to establish a gaseous fluid communication between the cells of the cellular component and the front face of the facade component to attenuate the noise produced by the flow of gaseous fluid on the surface of the front face of the facade component , characterized in that the multiplicity of holes (31) passing through the facade component is in the form of a network of holes which pass through the facade component from its rear face to its front face, with a predetermined direction of the holes, inclined to normal on the front side.   37. Panel according to any one of the claims.   cations 30 to 35, characterized in that the multiplicity of holes (31) passing through the facade component is in the form of a network of holes which pass through the facade component from its rear face to its front face, with a predetermined direction of the holes, inclined   compared to normal on the front panel.   38. Panel according to either of the claims   cations 36 and 37, characterized in that the inclination of the holes (31) is chosen so as to form flow paths in the structure defining cells,   which optimize the attenuation performance of the panel.   39. Panel according to claim 38, character-   rised in that the facade component (36) is a multi-ply structure comprising a plurality of overlapping ply elements, and that the holes are inclined so as to compensate for the structural weakness of the multi-ply structure in the region of holes.   40. Panel according to any one of the claims.   cations 36 to 39, characterized in that the panel (15, 16, 17) is arranged so as to be subjected to a stream of gaseous fluid on the surface of the front face of the facade component with a predetermined direction of the stream of fluid , and that the predetermined direction of the holes (31) has a component along the front face of the facade component, which extends in the same direction as the   predetermined direction of fluid flow.   41. Panel according to either of the claims   cations 36 and 37, characterized in that the holes (31) are inclined so that their tendency to be obstructed by debris entrained by the stream of gaseous fluid on the   front of the facade component is reduced.   42. Panel according to any one of the claims   cations 30 to 41, characterized in that the holes (31) of the network have drilling patterns which reduce blockage by debris which is deposited by the   gas current flowing in the holes.   43. Noise reduction panel, which includes   door: - a cellular component (35) which comprises: a front face and a rear face and a wall structure defining cells (37), which defines a multiplicity of cells between the front face and the rear face and which ends in end portions situated on the front face of the cellular component, and - a front component (36) which comprises a front face and a rear face, extends across the ends of the cells of the cellular component on the front face of this component, the rear face of the facade component being adjacent to the front face of the cellular component, and is provided with a multiplicity of holes (31), which   pass through the facade component from the front side to   than on the rear face to establish a gaseous fluid communication between the cells of the cellular component and the front face of the facade component to attenuate the noise produced by the flow of gaseous fluid on the surface of the front face of the facade component , characterized in that no hole is provided at the locations of the facade component, which, when the panel is assembled, are contiguous to the end portions of the walls of the   wall structure defining cells.   44. Panel according to any one of the claims   cations 30 to 42, characterized in that the wall structure defining cells includes walls which define the multiplicity of cells and which terminate in end portions on the front side of the cell component, and no holes are provided at locations of the facade component which, in the assembled panel, are contiguous with the end portions of the walls of the structure with cell walls.   45. Panel according to any one of the claims.   cations 30 to 44, characterized in that the holes (31) of the network have drilling configurations which reduce blockage by debris deposited by the gas current passing through them.   46. Noise reduction panel, which includes   takes: - a cellular component (35) which comprises: a front face and a rear face and a wall structure defining cells (37), which defines a multiplicity of cells between the front face and the rear face, and - a component facade (36) which comprises a front face and a rear face, 15. extends across the ends of the cells of the cellular component on the front face of this component, the rear face of the facade component being adjacent to the front face of the cellular component, and is provided with a multiplicity of holes (31), which   pass through the facade component from the front side to   than on the rear face to establish a gaseous fluid communication between the cells of the cellular component and the front face of the facade component to attenuate the noise produced by the flow of gaseous fluid on the surface of the front face of the facade component characterized in that the facade component (36) is in the form of a fiber reinforced composite article which comprises a matrix component (136) and a fiber reinforcement component (137) incorporated in the interior of the matrix component, the fiber reinforcing component having fiber arrays, in which the fibers of each array (139-141) extend in a predetermined direction in the matrix, and the predetermined direction of the fibers in each array differs from that of the fibers of each of the other networks, and the holes (31) contained in the facade component are formed so as to have a polygo cross section nale provided with sides arranged parallel to the predetermined directions of the fibers of the networks.   47. Panel according to any one of the claims   cations 30 to 45, characterized in that the facade component (36) is in the form of a fiber-reinforced composite article, which comprises a matrix component and a fiber reinforcement component incorporated inside the matrix component, the fiber reinforcement component (136) having fiber arrays, in which the fibers of each network extend in a predetermined direction in the matrix, and the predetermined direction of the fibers in each network differs from that of fibers of each of the other networks, and the holes (31) contained in the facade component are formed so as to have a polygonal cross section provided with sides arranged parallel to the predetermined directions of the fibers of the networks.   48. Panel according to either of the claims   cations 46 and 47, characterized in that the reinforcing fiber networks (139-141) include first, second, third and fourth fiber networks whose   fibers extend in directions 0, 90 and +45 and -     and that the holes in the facade component have an octagonal cross section whose sides are   parallel to the directions of the fibers.   49. Panel according to claim 48, character-   rised in that the reinforcing fiber networks (139-141) comprise first and second fiber networks, of which   fibers have directions 0 and 90, or +45 and -     and that the holes formed in the facade component are produced so as to have a polygonal cross section with four sides, the sides of which are   parallel to the directions of the fibers.   50. Panel according to any one of the claims.   cations 30 to 49, characterized in that the panel (15, 16, 17) further comprises a reinforcing component (34) which   extends over the back side of the cellular component.   51. Noise reduction panel manufactured according to   any one of claims 1 to 29.   52. Propulsion power unit for aircraft, comprising an engine body and a nacelle structure surrounding the body and defining a fan duct having inner and outer walls, characterized in that one or each of the walls of the duct of fan is formed by or includes a noise attenuation panel according to any of claims 30 to 51.   53. Propulsive power unit for aircraft, comprising a gas current duct, characterized in that it includes a noise attenuation panel according to   any one of claims 30 to 51.